Catalysts for purifying vehicle exhaust gas and the like are composed of a catalytic metal such as platinum, palladium, or rhodium, and a co-catalyst for enhancing the catalyst action of such metal, both supported on a catalyst support made of, for example, alumina or cordierite. The co-catalyst material absorbs oxygen under the oxidizing atmosphere and desorbs oxygen under the reducing atmosphere, and functions to optimally maintain the air/fuel ratio so that the catalyst for exhaust gas purification can efficiently purify noxious components in exhaust gases, such as hydrocarbons, carbon monoxide, and nitrogen oxides.
Efficiency of a catalyst for purifying exhaust gas is generally proportional to the contact area between the active species of the catalytic metal and exhaust gas. It is also important to maintain the air/fuel ratio at optimum, for which the pore volume of a co-catalyst should be made larger to maintain oxygen absorbing and desorbing capability at a high level. However, a co-catalyst, such as cerium-containing oxides, is apt to be sintered during use at high temperatures, e.g., for exhaust gas purification. This results in reduction of its pore volume, causing aggregation of the catalytic metals and decrease in the contact area between exhaust gas and the catalytic metals, which leads to reduction of efficiency in purifying exhaust gases.
In the light of the above, for improving the heat resistance of cerium oxide, Patent Publication 1 proposes a method of producing a cerium composite oxide containing cerium and other rare earth metal elements. The method includes the steps of: forming a liquid medium containing a cerium compound; heating the medium at a temperature of at least 100° C.; separating the precipitate obtained at the end of the preceding step from the liquid medium; adding thereto a solution of a compound of rare earth other than cerium to form another liquid medium; heating the medium thus obtained at a temperature of at least 100° C.; bringing the reaction medium obtained at the end of the preceding heating step to a basic pH to obtain a precipitate; and separating and calcining the precipitate.
The composite oxide obtained by this method is described to have a porosity of at least 0.2 cm3/g provided by pores having a diameter of at most 200 nm, after calcining at 1000° C. for 5 hours.
However, the largest porosity provided by pores having a diameter of at most 200 nm of the composite oxides disclosed in the specific examples in Patent Publication 1, is 0.24 cm3/g after calcining at 1000° C. for 5 hours, and the porosity of this composite oxide after calcining at 900° C. for 5 hours is 0.25 cm3/g provided by pores having a diameter of at most 200 nm. Thus further improvement is demanded.
Patent Publication 2 proposes, for the improvement of thermal stability of cerium oxide (ceria), a composition containing ceria and from about 5 to 25 mole % based on the moles of ceria of a ceria stabilizer selected from the group consisting of La, Nd, Y, and mixtures thereof. This composition is described to be prepared by mixing a ceria precursor with from 5 to 25 mole % of a ceria stabilizer selected from the group consisting of La, Nd, Y and mixtures thereof; forming an intimate mixture of the ceria precursor and the ceria stabilizer by either evaporation of the mixture of the preceding step or precipitation of the mixture of the preceding step as a hydroxide or a carbonate; and calcining the resulting intimate mixture.
However, as a property of the resulting composition, stabilized ceria, Patent Publication 2 is silent about the volume of pores with a diameter of not larger than 200 nm after calcination at 900° C. for 5 hours. Besides, the method disclosed in this publication does not provide a composite oxide having a larger volume of pores with a diameter of not larger than 200 nm after calcination at 900° C. for 5 hours, than the composite oxide taught in Patent Publication 1.